The present invention relates to a mobile radio communications system which realizes public communication networks by utilizing mobile stations.
Conventionally, personal handy phone systems (PHS) regarded as a type of mobile radio communications systems has a problem that a cell, i.e., a radio zone covered by a base station is so small that a handover operation for switching from one cell to another is frequently performed while a user making a call using a mobile station is moving at a high speed, whereby the call is temporarily interrupted for several seconds every time the handover operation is performed, thus largely degrading communication quality.
Communications systems for a personal handy phone system have been described in an article "Trends of Digital Cordless Telephones" (Transactions of the Institute of Electronics, Information and Communication Engineers, Vol. 77, No. 6, pp 636-642) about a concept of technical specifications for the systems.
In a personal handy phone system for public communications, each mobile station, when moving from one radio zone to another, automatically re-initiates a call in the destination radio zone, thus autonomously realizing the handover operation only by a normal call operation, without any particular intervening handover related operation from the network side. The first edition of the second-generation cordless telephone system standard (RCR STD-28) published by Radio Wave System Development Center Foundation (RCR) defines that the personal handy phone systems for public communications shall employ the above-mentioned re-call handover operation scheme.
The employment of the re-call handover operation scheme enables mobile stations to switch radio zones by using only a normal call operation without any particular operational intervention from the network side, thus providing a simplified handover operation. However, a mobile station needs to re-initiate a call in a destination radio zone every time it performs a handover operation as it moves from one radio zone to another, so that communication is interrupted during a handover operation for the time period required for the re-initiation which may cause a delay for up to several seconds. Since radio zone coverage is quite small in the personal handy phone system, if a call is being made while moving at a high speed, the handover operation will be repeated so frequently that interruptions of a call, caused by the handover operation, will occur at short intervals to an innegligible degree.
Assume for example that a mobile station is moving at 60 kilometers per hour (km/h), and a radio zone covered by a base station has a diameter of approximately 100 meters (m). A simple calculation based on these assumed values shows that the handover operation is performed every six seconds, and a call is interrupted for several seconds at each handover operation, thus presenting a problem that a precise communication quality cannot be ensured.
FIG. 1 illustrates an example of the configuration of a conventional personal handy phone system for showing how the position of a communication slot transits on signal lines (500) and on radio transmission channels (300a)-(300c) when the handover operation is performed each time a mobile station (100) moves from one radio zone to an adjacent radio zone.
In the conventional personal handy phone system as illustrated, when the mobile station (100) under communication is moving from one radio zone (200a) to an adjacent radio zone (200b) and further to another radio zone (200c) covered by respective base stations (400a)-(400c) in succession, the mobile station (100) re-initiates a call to a mobile communications exchanger (600) through a corresponding base station (400a)-(400c) forming the radio zone (200a)-(200c), each time the mobile station (100) enters a new radio zone (200a)-(200c), to carry out the handover operation. Since a call is again set each time the mobile station (100) moves from the radio zone (200a) to (200b) and from (200b) to (200c), the position of the communication slot for this call is different on the signal lines (500) and on the radio transmission channels (300a)-(300c) in the respective base stations (400a)-(400c).
A technique for switching a pertinent radio zone without performing the handover operation as a mobile station moves from one radio zone to another may be represented by a communications system adapted by the Japanese Shinkansen for its transportation service and public telephone service. However, since the network side detects movements of respective trains from one radio zone to the next and realizes the switching of the radio zones by the control performed on the network side, a movement detecting means and a radio zone switching control means are required on the network side. While the provision of these means is effective in a system having a relatively small number of base stations, a system employing a concentrated control scheme for the radio zone switching, or the like, it is difficult to apply the same to the personal handy phone system which includes a large number of base stations, each of which autonomously performs the radio zone switching.